Abnormal cells in the body are known to selectively absorb certain dyes delivered to a treatment site to a more pronounced extent than surrounding tissue. Once presensitized, the cancerous or abnormal cells can be destroyed by irradiation with light of an appropriate wavelength corresponding to an absorbing wavelength of the dye, with minimal damage to surrounding normal tissue. This procedure, which is known as photodynamic therapy (PDT), has been clinically used to treat metastatic breast cancer, bladder cancer, head and neck cancers, and other types of malignant tumors.